Method for releasing cables from an attached well tool and apparatus for exercising the method

ABSTRACT

In a method for releasing cables ( 2 ) from an attached well tool, where the cutting and the time for cutting can be decided from the surface above the well, and where a cable cutter ( 6 ) is arranged between the main part of a cable and a well tool, the cable cutter being controlled by an electronic timer, it is suggested that at least three parameters are used for controlling the cutter, e.g. a pulling force exerted by the cable on the well tool, the pressure in the well location, and time.

TECHNICAL FIELD

The present invention relates to a method for releasing cables from anattached well tool, where a cable cutter is arranged between the mainpart of a cable and a well tool, said cable cutter being controlled by atimer. The invention relates also to an apparatus for exercising themethod.

PRIOR ART TECHNIQUE

A well bore apparatus being adapted for being releasably coupled tosuspension cables is disclosed in the U.S. Pat. No. 4,237,972 in thename of C. P. Lanmon. In this prior art the inventor explains the ideaof cutting the cable when the tool has fastened in the well. The basicidea is to avoid a long piece of a cable being left and destroyedbetween the tool and the fishing tool. Therefore, it is necessary to cutthe cable close to the tool, and the cutting device is in this prior artcontrolled by a time delaying retaining means for preventing theactuation of the cutter, said retaining means having achemically-corrodible element within a chamber, where a corrosivechemical can act upon the chemically-corrodible element. It is evensuggested, that the chemically-corrodible element is a metal and thecorrosive chemical is either an acid or a caustic. The corrosive elementwhen released by the chemical degradation fires an explosive driving thecutting blade. To activate the cutting means, before the apparatus islowered into a well bore a corrosive chemical is released in theapparatus for causing the failure of a restraining means only after theelapse of a predetermined time interval that is ordinarily more thansufficient for carrying out the intended operation of the apparatus.This has, however, the drawback, that the apparatus is time sensitive inrelation to delays in the well operation, and on the other hand, thatmuch time will be spent waiting, when the apparatus has stuck in thewell bore.

In U.S. Pat. No. 3,073,388 another actuating means is used for thecutter. This actuating means is driven by explosives disposed in acasing means and controlled by firing means and a timing mechanismoperatively connected to the firing means. The timing means are presetbefore being sent down the well and are initiated by positioning of thecasing means at a remotely inaccessible location in the well bore. Thistiming means is not initiated until the positioning and much time will,also in this case, be spent waiting for the firing of the explosives. Aquick release through an early cut is thus excluded. On the other hand,if, for some reason, the tool is released again, there is no way to stopthe timing means and the cable will thus unavoidably be cut after thelapse of the preset time.

A normal procedure for using tools in a well bore is to send down thetool suspended in a cable. The cable is capable to carry a certain loadbefore being destructed by the tensile stress. The tool has a mass farbelow the limiting tensile stress of the cable. If the tool one wayanother is stuck in the well bore, it is important to release the tooland pull up the cable, as exerting further tensile stress will pass theyield point of the cable material thereby permanently reducing the crosssection of the cable and its possible ability to carry large electricalcurrents.

In cases, where the cable is also an electric wire, the electric currentis depending on the tensile stress as the tensile stress reduces thecross section of the wire or the single strands and is therebyincreasing the energy loss through the cable. Thus, if the cable isstretched too much, it will introduce a large specific resistance orshort-circuit the different cords.

SUMMARY OF THE INVENTION

Therefore, it is a primary object of the present invention to provide amethod for the cutting of the cable, where the cutting and the time forcutting can be decided from the surface above the well. To this end, theinventor has suggested that at least three parameters are used forcontrolling the cutter, e.g. a pulling force exerted by the cable on thewell tool, the pressure in the well location, and time, where the timeris working electronically. The term “cutting” is to be interpreted in abroad sense to include any intended disconnection between the cable andthe well tool, except rupture. Preferably, the temperature can also beused as a controlling parameter. The electronic timer is controlling acutter of any appropriate type being driven by explosives or any othermeans. With such control of the reset of the timer the cutting can onlybe performed, when the cable for some time is without any current andthe conditions given by the surroundings are according to the set valuesof the pressure, the force, or temperature, respectively. Further, bybeing able to reset the timing device by changing the current throughthe cable, the cutting can be avoided at any time before the cutting isperformed.

In a preferred embodiment the timer is reset from currents in the cablethrough the use of an interface pick-up. Thereby, a steady detection ofthe currents in the cable will be possible for both large currents, i.e.measured in A or kA, or smaller currents as measured in mA.

If the cable incidentally has been destructed by increasing the tensilestress beyond the rupture limit, no current can be guided through thecable. The location of such rupture is normally closer to the surfacethan to the tool, as the upper part of the cable also is carrying theload of the lower part of the cable. The cutting will therefore takeplace after the actual preset delay. This is just a security measure, sothat the cable sooner or later will be cut. In order to avoid suchdestruction the inventor has suggested that the pulling force exerted bythe cable on the well tool is limited by a mechanical unit. It isfurther suggested that the activation of the limitation unit can be usedfor triggering the timer.

Sometimes, the delay will be too long, if the tool must be retrieved ina short time. To this end, the electronic timer—in an advantageousembodiment—can be controlled by pulses smaller than the reset pulses,thereby making it possible to the operative crew to remotely control thepreset timer delay and the triggering of the cutter. In an advantageousembodiment a capacitor can be arranged between the cable and ground sothat small pulses or their flanks can be directed to the timer, withoutseriously influencing the circuits for the larger working currents..

To exercise the invention it is suggested to use an electronic timercomprises a receiver for receiving reset signals generated by thecurrents in the cable, an auxiliary inlets for signals from sensors forphysical parameters, and an outlet for the control of a cutter control.The physical parameters could be the pressure, temperature, or tensilestress. When these parameters exceed a preset value or leave apredetermined range, a signal is given to the timer, which—after thepreset timer period—fires the cutter control and thereby disconnects thecable from the tool. The cutter control can comprise explosives or othercutting effective arrangements to secure the cutting of the cable, whenneeded. Such cutting effective arrangements could be springs workingdirectly with knives, scissors or shears.

More embodiments of the subject matter invention are disclosed in thefollowing detailed description of the invention, where an explanation isgiven using an example of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described more clearly in a preferred embodiment shownin the drawing showing an example of a diagram of a circuit for cuttingthe cable.

DETAILED DESCRIPTION OF THE INVENTION

In the drawing an electronic timer device 1 is in connection with thecable 2 through a pick-up 3. Under influence from not directly shownsensors information about the pressure and the force exerted on thecable is directed to the timing device 1, which—provided the pressureand the force are according to the set values—after a period with nocurrents in the cable will activate the explosives control 4. This cantake place at any time after the tool has stuck in the well Even if theforce exerted on the cable has exceeded the limit, so that the cable hasbeen destructed, it might still be possible to guide pulses down to thecontrol circuit for the electronic timer. This is caused by thecapacitor 5 arranged between the cable 2 and ground so that small pulsesand their flanks can be directed to the electronic timer, withoutresetting the electronic timer circuit. For influencing the reset of theelectronic timer, larger working currents are needed.

When the right conditions for firing the explosives are present, i.e.the tensile stress is above a certain limit, the pressure is likewiseabove the preset limit, the temperature might be above the preset limit,and the timer has been allowed to count down, the explosives control 4will fire and activate the cutter 6, which thereby will cut the cableclose to the attached well tool so as to leave as little cable aspossible with the attached well tool. This measure should secure a safefishing of the tool, as earlier such left-over of the cable could bothbe a waste of the cable and impede the fishing of the tool.

1. Method for releasing cables (2) from an attached well tool, wherein acable cutter (6) is arranged between the main part of a cable and a welltool, said cable cutter being controlled by an electronic timer, themethod comprising: using at least three parameters for controlling thecutter: a pulling force exerted by the cable on the well tool, thepressure in the well location, and time; and resetting the timer fromcurrents in the cable (2) through the use of an interface pick-up (3).2. Method according to claim 1, the method further comprising: usingtemperature as a controlling parameter.
 3. Method according to claim 1,wherein the timer (1) is also responsive to pulses of currents, themethod further comprising: making it possible for an operative crew toremotely control preset parameter values of force, pressure,temperature, or timer delay and thus a triggering of the cutter (6). 4.Method according to claim 3, the method further comprising: controllingthe timer by pulses smaller than reset pulses, wherein the pulses aregiven by the operative crew from the surface to remotely control thepreset parameter value of timer delay and the triggering of the cutter.5. Method according to claim 4, characterised in that the preset valuesfor the pressure tensile stress or temperature can be changed from theoperative crew.
 6. Method according to claim 4, characterised in that acapacitor is arranged between the cable and ground.
 7. Apparatus forreleasing cables (2) from an attached well tool, wherein a cable cutter(6) is arranged between a main part of a cable and a well tool, theapparatus comprising: an electronic timer that controls the cablecutter, the electronic timer comprising a receiver (1) for receivingreset signals generated by currents in the cable (2), auxiliary inletsfor signals from sensors for physical parameters, an outlet for controlof a cutter control (4), and a pick-up (3), the electronic timer inconnection with the cable through the pick-up, wherein the pick-upgenerates reset signals by a steady detection of the currents in thecable, to which the tool is attached.
 8. Apparatus according to claim 7,characterised in that the receiver (1) further has at least one circuitcontrolled by small pulses from the cable (2), which is connected toground through a capacitor (5).
 9. Apparatus according to claim 8,characterised in that a pulse generator on the surface is connected tothe cable (2).
 10. Apparatus according to claim 7, characterised in thatthe cutter control (4) is connected to the cable cutter (6). 11.Apparatus for releasing cables (2) from an attached well tool, wherein acable cutter (6) is arranged between a main part of a cable and a welltool, the apparatus comprising: an electronic timer that controls thecable cutter, the electronic timer comprising a receiver (1) forreceiving reset signals generated by currents in the cable (2),auxiliary inlets for signals from sensors for physical parameters, anoutlet for control of a cutter control (4), and a pick-up (3) whichgenerates reset signals; and the receiver having at least one circuitcontrolled by small pulses from the cable (2), which is connected toground through a capacitor (5).
 12. Apparatus according to claim 11,characterised in that a pulse generator on the surface is connected tothe cable (2).
 13. Apparatus according to claim 11, characterised inthat the cutter control (4) is connected to the cable cutter (6).